Ice: 'Bad forecast miss'
Freezing rain ambushes motorists -- and weathermen.
Motorists weren't the only ones ambushed by Sunday morning's attack of freezing rain – so were the meteorologists.
By the time the National Weather Service officially hoisted the ice advisories, the lacquering already was well under way.
The long and short of it: The air was a few degrees colder, and the rain came a few hours sooner, than expected.
Local winter veterans are aware that an imperceptible accumulation of ice can be far-more dangerous and disruptive than a foot of snow. And the more imperceptible, the more dangerous it is.
The groundwork for the Sunday morning disaster was laid Saturday night with a prosaic forecast problem.
Clouds didn't develop as quickly as expected, and that allowed for more cooling than expected, said Sarah Johnson, a lead forecaster at the weather service's Mount Holly office.
Clouds tend to trap in heat that otherwise can escape into space. The temperature fell to 27 by 9 p.m. Saturday, and to 24 overnight.
It began rising toward dawn, but by then the frozen ground was ripe for the cold rain that snuck in a few hours ahead of schedule.
When the rain started, temperatures were at or just below freezing in Philly, and in the upper 20s in Pottstown.
Just before daybreak, Gary Szatkowski, Johnson's boss in Mount Holly, sent out a Tweet proclaiming, "High impact icing event paralyzing much of the region this morning. Don't travel until it warms up. And, yes, this was a bad forecast miss."
And it was. Computer model forecasts on temperature were perilously off the mark.
With the 10 p.m. Saturday forecast discussion, the weather service did mention the potential for icing in some areas but that it was "not planning to issue any headlines at this time given the uncertainty with the timing of precipitation."
When the rain started, however, temperatures were at or just below freezing in Philly, and in the upper 20s in Pottstown.
The weather service issued a freezing-rain advisory at 7 a.m. Sunday, but by then motorists didn't need a weatherman to know that this was a dangerous morning on which to be driving.